Transmission of microwave frequency signals over fiber, termed in the industry as “radio-over-fiber” (RoF), has provided several advantages over wireless transmission. For example, a microwave signal propagated through an optical fiber can be transported over longer distances without signal degradation in comparison to a wirelessly-transmitted microwave signal that often requires line-of-sight propagation and suffers degradation due to atmospheric conditions. Typically, such a microwave signal is used as a sub-carrier signal for carrying data related to various applications. For example, in one such existing art, an RoF technique has been employed for transporting voice signals between a central office serving land-line phone customers, and a cellular base station serving cellular phone customers. Unfortunately, existing RoF systems such as this suffer from various constraints.
To illustrate at least one such constraint, attention is drawn to FIG. 1, which shows a prior-art communications system 100 that communicatively couples Public Switched Telephone Network (PSTN) 130 to cellular network 140. PSTN Central Office (CO) 105 provides telephone services to various telephone subscribers 101, 102, and 103. In one exemplary implementation, the telephone voice signals from these multiple customers are multiplexed together to create digital data that is transported over, what is known in the industry, as an Optical Carrier (OC). The OC may be operated at various bit-rates and as such is designated “OC-x” in FIG. 1. These optical bit-rates correspond to industry-wide standards that are generally known to persons of ordinary skill in this art. The OC-x digital data in PSTN CO 105 is coupled to a Synchronous Optical Network (SONET) laser 106, which utilizes baseband amplitude modulation (AM) to generate an AM modulated optical signal.
The AM modulated optical SONET signal is transmitted through optical fiber 110 to another PSTN CO 115, wherein it is directed to an Optical-to-Electrical (O/E) converter 117. O/E converter 117 typically employs a photodiode for carrying out conversion from the optical to the electrical domain. The converted electrical signal is coupled into an electronic AM-FM converter 118 wherein the signal is suitably de-modulated from the AM format into a baseband signal, which is then re-modulated into a frequency modulated (FM) signal that is in conformance with a signaling format of cellular network 140.
The FM electrical signal is coupled into electrical-to-optical (E/O) converter 119 for creating an RoF optical signal that is then transmitted via optical fiber 120 to cellular base station 125. Circuitry at cellular base station 125 de-multiplexes the multiple telephone calls carried in the RoF FM signal and routes individual voice signals to the appropriate cellular phone customers 126, 127, and 128.
As can be understood from the description above, PSTN CO 115 incorporates circuitry that converts an AM signal of a first format to an FM RoF signal of a second format by employing conversion between optical and electrical domains. The conversion circuitry introduces inefficiency in the form of potential signal degradation due to factors such as insertion loss and impedance mismatch, and further introduces reduced equipment reliability arising from the number of components used for carrying out the double conversion process (O/E followed by E/O).